Field of the Invention
The invention relates to an apparatus and an associated method for a synchronous demodulation of phase shift keyed signals.
A receiver with synchronous demodulation has been described in Frowin Derr, Coherent optical QPSK intradyne system: Concept und digital receiver realization, IEEE J. Lightwave Technology Vol. 10, 1290-1296, (1992). The carrier phase is ambiguous according to the number of transmitted phase values, twofold in the case of binary phase shift keying, abbreviated BPSK, and fourfold in the case of quaternary or quadrature phase shift keying, abbreviated QPSK. For a realization at the lowest possible cost of such and similar receivers it is desirable to sample the available electrical signals at the symbol clock frequency or a multiple thereof. Especially in such digital receivers it is difficult to avoid ambiguity errors of the recovered carrier phase. After sampling at the symbol rate the sampled values can be demultiplexed, i.e. distributed into M data streams, the symbol rate of each of which is 1/M times as large as the original symbol rate. However, if all signal processing operations are performed at the 1/M-fold of the original symbol rate then the problem occurs that the phases of the recovered carrier occuring in the various demultiplex channels can carry ambiguity errors. For BPSK an error of the size π can occur. For QPSK errors of the size π/2, π or 3π/2 can occur. This causes errors during data recovery. At best, narrow linewidths of the employed lasers can avoid this, but lasers with narrow linewidths require external resonators, which is inconvenient and leads to high cost.